Can you wear Army unit medal on Navy uniform?

Yes you can. According to Chapter 5 artical 5301-5319 of the Navy Uniform Regulation any military decoration earned while serving in the army may be worn behind it's Navy equivilent (i.e. the navy equivilent has precidence over other branches). The only acception to this is the Army Service Ribbon which cannot be worn on a Navy uniform. For further questions see before mentioned regulation or contact Awards and Special Projects Office (OPNAV 09B33), Navy Yard, Building 36, First Floor, Washington DC 20350.